The present invention relates to an image forming apparatus wherein a toner image for each color is formed when each of plural rotating photoreceptor drums is given exposure and is developed, and a color image is formed when the toner image for each color is transferred onto a sheet through an intermediate transfer member.
In the image forming apparatus of this kind, color registration deviation caused in superposition of toner images for respective colors takes place because of various factors, and it is necessary to correct this color registration deviation. However, when correcting the color registration deviation by an exclusive sequence wherein an image forming job is interrupted, productivity of the image forming apparatus is lowered.
Under the aforesaid background, there is known a technology that is called “Inter-sheet color registration correction” for correcting aging color registration deviation caused by temperature rise in an apparatus, without lowering productivity of the image forming apparatus (for example, see Unexamined Japanese Patent Application Publication No. 2005-292760) The Unexamined Japanese Patent Application Publication No. 2005-292760 is purported to be capable of correcting color registration deviation accurately in the course of sheet feeding without interrupting an image forming job, by making a conveyance interval for sheets to be broader than a prescribed interval, then, by forming a mark for detecting color registration deviation at a position corresponding to a point between sheets on an intermediate transfer member, and, by detecting the mark for detecting color registration deviation with a sensor, and by calculating an amount of color registration deviation that has been detected by the sensor.
Further, there is known an image stabilizing control wherein a temperature of an optical element that guides a light beam for exposure scanning to a photoreceptor drum from a light source is detected, and copy operations are interrupted when a temperature difference between the present time and the past is large, and correction of the exposure scanning position for the light beam is carried out (see Unexamined Japanese Patent Application Publication No. 2007-199211). The Unexamined Japanese Patent Application Publication No. 2007-199211 is purported to be capable of conducting the image stabilizing control at appropriated timing to maintain necessary image quality, and thereby to reduce waiting time for a user.
Further, with respect to forming of a mark for detection of color registration deviation between sheets, there has been known an idea to change a length of a pattern of a color registration deviation detecting mark based on an inner temperature of an image forming apparatus that is a dominant cause for color registration deviation (see Unexamined Japanese Patent Application Publication No. 2008-175966). The Unexamined Japanese Patent Application Publication No. 2008-175966 is purported to be able to reduce the consumption volume of toner in the case of correction of color registration deviation, and to realize a reduction of an amount of time required for correction.
Now, color registration deviation caused in superposition of toner images for respective colors is generated in the direction that is in parallel with the conveyance direction for sheets (sub-scanning direction), and in the direction that is perpendicular to the sheet conveyance direction, and a type of a color registration deviation detecting mark to be formed varies, depending on the direction of the color registration deviation to be corrected.
A size in the sub-scanning direction of the color registration deviation detecting mark to be formed for correcting color registration deviation in the direction perpendicular to sub-scanning direction (main-scanning direction) is larger than that of the color registration deviation detecting mark to be formed for correcting color registration deviation in the sub-scanning direction, and an amount of toner to be needed is larger. Therefore, the time required for correction for correcting color registration deviation in the main scanning direction is longer than the time required for correcting color registration deviation in the sub-scanning direction, and toner in large quantities is not avoided.
Therefore, it is preferable that a color registration deviation detecting mark is formed for correcting color registration deviation in the main scanning direction only when color registration deviation in the main-scanning direction takes place, and a color registration deviation detecting mark for correcting only color registration deviation in the sub-scanning direction is formed when no color registration deviation in the main-scanning direction takes place.
The invention has been achieved in view of the aforesaid problems, and its object is to provide an image forming apparatus wherein the time required for correction for correcting color registration deviation is short, and the consumption volume of toner is less.